To each his own. I presume some might find aspergers or something else as a shield, as a way to have an excuse (I think this was mentioned).
However, lack of diagnosis doesn't mean that autistic traits are not present. What matters first and foremost is how the person feels inside and in relation to others. There may be many underlying conditions that can explain estranged feeling. The main question here is, when did the problems start? Somebody who had normal childhood and then normal adolescense, and suddenly becomes withdrawn certainly isn't autistic.
It is not plausible to search for evidence retrospectively to find symptoms because we tend to distort memories in order to fit present.
This was a problem for me... I never, until recently, thought about having anything with autism. I was certain I had either schizoid or shcizotypal PD. Then I remembered high school - no friends, and then elementary school - outcast. And it was precisely the early age that caused me problems because I, at that time, believed others were strange, not me.
Then I asked other who knew me from birth about myself:
''you started talking later but it were whole sentences''
''we couldn't teach you how to tie shoelaces''
''you were always special''
''you were hostile to other kids''
''I remeber how you were obsessed with that abstract nonsense sculputres in museum''
And so on, and so on. And then I started to suspect.
Anyway, I am sorry for such a long post. If something has my attention, I can drag it on, and on, and on...
The point is...self diagnosis can be relevant, I believe, if certain conditions are met.